This invention relates to process and materials for imprint lithographic applications and more particularly, to vinyl ether based coating compositions suitable for nanoimprint lithography.
Imprint lithography has emerged in various forms as a potential alternative to conventional photolithography because of its ability to print smaller features at low cost. UV-cure nanoimprint lithography is a variant of imprint lithography that is amenable to the resolution and overlay requirements necessary for the fabrication of advanced semiconductor devices. In UV-cure nanoimprint lithography, a low-viscosity photosensitive molding material is molded between a mechanically rigid template having a relief pattern and a substrate, and then is exposed to actinic radiation. The resulting hardened layer, having a three dimensional pattern, can for example be used as an etch mask to transfer the imprinted pattern into the substrate below. Other applications are also possible.
Imprint lithography requires low volatility and low viscosity resists to obtain high quality patterned films with uniform composition within the minimum possible cycle time. The composition of the curable material is of critical importance because its components affect the degree of cure, adhesion to the template surface, adhesion to the bottom surface, cohesive strength of the cured material, and the dimensional stability of the imprinted features. Vinyl ether based resists are attractive chemical systems for this purpose because they have low volatility, low viscosity, and also because these materials have very rapid cure rates. Vinyl ether based resist formulations can be cationically cured upon exposure to actinic radiation and the cure chemistry is not affected by free radical traps such as oxygen, thereby reducing the sensitivity of the process to ambient air. Vinyl ether based resists are sensitive to ambient base, however, and their reactions can be inhibited by water vapor.
Prior art cationically curable vinyl ether based imprint resist systems typically contain a difunctional silicon-containing vinyl ether crosslinker, a photoacid generator (PAG), an optional reactive monofunctional vinyl ether diluent, and a sensitizer/stabilizer additive such as 9-anthracene methanol. The silicon-containing difunctional vinyl ether crosslinkers are commonly used for etch barrier and reverse tone patterned bottom layer applications. Some prior art vinyl ether resist formulations that do not contain silicon-containing difunctional crosslinkers include aromatic vinyl ether formulations that are either 100% crosslinker without a diluent or include up to about 20% fluorinated vinyl ethers or other fluorinated non-VE compounds as the diluent. These types of formulations are often used for reverse tone bottom layer applications. Although initially thought to be beneficial for release of the cured resist from the nanoimprint template, these fluorinated components have been found to be highly volatile and to evaporate from the uncured resist formulation.
The PAG, which is typically a solid material, or other solid additives such as the sensitizer in the vinyl either resist formulations are typically chosen to be completely soluble in the crosslinker as well as the diluent. If the PAG or other solid additives such as 9-anthracene methanol are not soluble in the vinyl ethers or only slightly soluble, the solid material present in the resist will cause roughness and inhomogeneity in the cured material. Hard PAG or additive particles can potentially damage the surface of the template. Moreover, if the solids are removed by filtration little or no PAG (or solid additive) may be left in the formulation. Still further, it is possible that the PAG solids can precipitate out. In addition, a requirement for filtration to remove undissolved PAG solids will cause the composition of the formulation to be hard to control. There are contradictory reports on solubility of various components. Successful preparation of a Si-containing 80% crosslinker formulation with a PAG soluble in only one of the VE components has been described but subsequent work showed that the PAG used was not soluble in a similar formulation. In that work use of a mixture of 2 vinyl ethers with a PAG soluble in only one resulted in PAG precipitation on standing.
There are a number of problems with the prior art resist formulations for imprint applications. Formulations that include 100% crosslinker make very poor quality films in general. Micrographs of a cured film of the prior art compound diethyleneglycol divinylether are shown in FIG. 1. The resist formulation was imprinted manually and using an imprint tool commercially available under the trade name Imprio from Molecular Imprints, Inc. The resulting cured films are highly stressed, and tend to buckle and form fissures. The all organic films have a greater tendency to buckle, while occasionally some 100% and 50% crosslinker Si-containing resists can be of better quality. There is no guidance on use of vinyl ethers with functionality greater than 2 to make good films. The sensitizer/stabilizer and PAGs are not universally soluble in vinyl ethers, which limit the potential resist components. The preferred soluble PAGs such as CGI 1906 and 1907 are heavily fluorinated. The large degree of fluorination makes the PAG segregate to hydrophobic surfaces (e.g., the air interface) according to XPS measurements, thus limiting dispersion in the resist and making material uniformity hard to control.
Accordingly, there is a need in the art for improved vinyl ether resists suitable for imprint applications that provide cured films that are smooth and free of buckling, cracks, and pinholes.